The present invention relates to a process for cleaning the exhaust gas from a fusion reactor of exhaust gas components containing heavy hydrogen, the heavy hydrogen exhaust gas components comprising tritium and/or deuterium in their elemental forms and impurities which contain tritium and/or deuterium in chemically bound form, wherein the tritium and/or deuterium is released from its chemically bound form, and the released tritium and/or deuterium and the elemental heavy hydrogen are separated from the exhaust gas and returned into the fuel cycle.
The exhaust gas from the fuel cycle of a fusion reactor consists essentially of non-reacted fuel, tritium and deuterium in their elemental forms and, in addition helium, the "ash" from nuclear fusion reaction, it contains quite a number of other impurities whose concentrations attain only a few percent and which, partly, contain heavy hydrogen chemically bound to them. The majority of these impurities are carbon monoxide and hydrocarbons, such as methane. In addition, ammonia, carbon dioxide and water vapor usually occur at lower concentrations in the exhaust gas.
On account of its content of elemental and chemically bound heavy hydrogen (tritium and deuterium), this exhaust gas cannot be stacked directly to the atmosphere because the heavy hydrogen (tritium and deuterium) fraction must be separated beforehand. It is furthermore desirable to recycle tritium and deuterium into the fusion process.
Various techniques have been proposed for cleaning the exhaust gas of a fusion reactor. Processes and an apparatus for decontaminating exhaust gas of tritium and/or deuterium have been suggested by Kerr et al, "Fuel Cleanup System for the Tritium Systems Test Assembly: Design and Experiments", Proceedings of Tritium Technology in Fission, Fusion and Isotopic Application, Dayton, Ohio, Apr. 29, 1980, at pages 115 to 118. According to one process described by Kerr et al, the exhaust gas containing the impurities is first passed through an intermediate container, that is, a variable volume surge tank which is used to remove flow fluctuations and provide a constant feed pressure. The exhaust gas is then passed to a first catalytic reactor in which any free oxygen is reduced and combined with hydrogen at 450.degree. K. to form water. The exhaust gas is then sent to a molecular sieve bed at 75.degree. K. in which all impurities are adsorptively removed and are thus separated out from the exhaust gas. When the capacity of the molecular sieve bed is exhausted, it is heated to 400.degree. K. to desorb the impurities which are then sent to a second catalytic reactor in the form of an oxygen-supplying packed bed operating at 800.degree. K. where the impurities (e.g., ammonia and hydrocarbons) are oxidized into tritium- and/or deuterium-containing water and into tritium- and/or deuterium-free compounds, namely into CO.sub.2, N.sub.2 and Ar. The tritium- and/or deuterium-containing water then is frozen out at 160.degree. K., and thereafter the frozen water is periodically vaporized. The vapors are fed into a hot uranium metal bed which acts as a getter and which at 750.degree. K. transforms (reduces) the water into D- and/or T-containing hydrogen and stable UO.sub.2. In lieu of the reduction by means of the uranium metal bed, Kerr et al state that the reduction can also be carried out with the aid of an electrolytic cell when such a cell becomes available.
Kerr et al also describe a process based on hot uranium metal getters. In this process, the exhaust gas, after leaving the variable volume surge tank, enters a primary uranium bed operating at 1170.degree. K. In this bed, impurities are removed by chemical reactions that form uranium oxides, carbides, and nitrides. The inert argon, with traces of the other impurities, passes through the primary uranium bed and is sent to a molecular sieve bed as in the above-described process. The regenerated argon, with a small amount of tritium, is sent from the molecular sieve bed to a titanium bed, at 500.degree. K., which collects DT and passes on an argon stream containing only tenths of a ppm of DT. Kerr et al state that a disadvantage of this process is that operating temperatures of 1170.degree. K. cause permeation and material problems.
Kerr et al also describe the use of palladium diffusers, and state that they have numerous disadvantages including the need for elevated pressures, reported brittle failures during temperature cycling, reported poisoning by ammonia and methane, and the fact that they can not produce an impurity stream free of hydrogen isotopes.
P. Dinner et al, "Tritium System Concepts for the Next European Torus Project", Fusion Technology, Volume 8, No. 2, Part 2, pages 2228-2235, September 1985 (2nd Meeting on Tritium Technol., Dayton, Ohio, 1985) use for the purpose of cleaning the exhaust gas of a fusion reactor a combination of a high-temperature getter and a palladium/silver membrane. The drawback associated with this process and the process of Kerr et al that employ getter beds is that the getter bed must be operated at high temperatures (up to 900.degree. C.). Both material problems and problems resulting from losses due to permeation of tritium may occur during the processes of Kerr et al and Dinner et al. The replacement of the getter bed, which is necessary periodically, gives rise to safety problems due to the fine pyrophoric dust released and the fact that the getter bed must be disposed of as radioactive waste.
Another means of cleaning the exhaust gas of a fusion reactor according to P. Dinner et al, (talk during the 2nd Meeting on Tritium Technol., Dayton, Ohio, 1985) consists in a combination of high-temperature getter, oxygen releasing fixed bed for catalytic oxidation. The associated drawback is that the separated tritium is obtained as water whose radiotoxicity is greater by orders of magnitude compared to gaseous tritium, that the process requires a great number of process steps, and that the oxygen releasing fixed bed has to be operated at 500.degree. C.
At this temperature level there is the danger that the catalyst sinters and becomes ineffective and that an explosion might be caused by an uncontrolled release of oxygen.
In DE-OS 36 06 316 and in DE-OS 36 06 317, processes are described for cleaning the exhaust gas from fusion reactors wherein an oxygen releasing fixed bed, a metal bed as the "getter," and a palladium or palladium-silver membrane are used. DE-OS 36 06 317 further discloses the use of a Ni-catalyst. These processes are subject to the drawbacks which are associated with the operation of the oxygen releasing fixed bed.